Tuning the Fatman


I doubt anyone is ever going to need this file, but since I’m doing this work, I might as well record it to have something to go back to.


(My set-up. The A-300 is on the floor to the right, propped up against the table.)

The PAiA Fatman has been around at least since 1999 (copyright data printed on the main circuit board), and the instructions really haven’t been updated since then. So, I’m assuming that I’m in the minority of new customers that don’t know how to tune it based on “C” and “octaves”. The instructions for tuning the synth tell you to press the lowest “C” key on your keyboard, set the pitch control to match a reference note from a tuned keyboard, and then tweak the trim pots to properly adjust VCO2. Then, go to the highest “C”, repeat the process for 4 octaves up, do the same thing for the “C” notes in the middle for the middle octaves, and finally either do the entire process a couple times to get VCO1 and VCO2 in perfect harmony at both ends of the scale, or not. While there’s a suggestion for using an oscilloscope or frequency counter instead, there’s no mention of the actual frequency numbers that are needed for each “C”. Since I don’t have a scope, frequency counter or reference synth, I’d look to be out of luck.


(Japanino in freq. counter mode.)

However, I do have the Gakken version of the Arduino micro-controller, the Japanino, with the LCD shield. I can hack together a very simple sampling o-scope and a frequency counter, as long as I’m careful about not applying too large a signal to the Japanino analog in pins. I could save myself a bit of worry by replacing the 3-AA battery pack with a 4-AA pack and get Vcc up to 6V (some of the signals in the Fatman are 7V) and then put a potentiometer in series with the analog in pin as an amplitude control. But for right now, that’s not really necessary.


(Jumpers on the pot. controls are running to the Japanino.)

The Fatman only recognizes MIDI notes in the range from 36 to 84. If you look at a MIDI to frequency table, you’ll see that the lowest usable key is generally note 21, which is A0, or 27.5 hz. Anything below this is really hard to hear with the human ear. C1 is note 24, at 32.7 Hz. For the Fatman, the lowest key is MIDI note 36, which is C2, at 65.4 hz. The PAiA tuning instructions refers to this as “C0”. The highest used note is 84, “C6”, at 1046.5 hz. This then gives us the other C’s as:

C# Note# Hz
C0 36 65
C1 48 130
C2 60 261
C3 72 523
C4 84 1046

We don’t need to be dead-on with the frequencies, “close enough” is good enough. PAiA suggests as an alternative to a reference synth to use the online tuning fork. I think having the freq. counter sketch on the Japanino will be fine. What does make a big difference is having Offset at the middle of the range, pointing to Unison, and most of the other controls also at midrange just to get an audible signal to the speakers.


(Finished Fatman all boxed up.)

The Fatman doesn’t have its own MIDI keyboard, so I’m using my Roland A-300 Pro. This is a 32-key MIDI controller with +/- octave shift keys. The easiest approach is to hook the A-300 to the Fatman MIDI IN, then press the -Octave button on the keyboard and depress the lowest keyboard key until there’s no sound out from the Fatman. Work up the keyboard keys until you get the first playable note, and that’s going to be C0 for our purposes. Follow the tuning instructions for the Fatman. Then count up 12 white and black keys to C1, and adjust the R13 trimmer to give a frequency of roughly 130 hz. Use the +Octave button to shift the keyboard and press the notes to get the highest playable one. This will be C4, and count down 12 black and white keys to get to C3, and adjust R18 to 520 hz. Count down 12 keys to C2 and adjust with R21, and again to C1 and adjust R24. The last step is to set Offset to Unison and adjust R42 so that VCO1 and VCO2 are at the same frequency.


(Back panel. The only thing I wish were different is having an AC plug-in jack, rather than having the power adapter permanently wired to the box. Note the Pitch, Vel. and Gate RCA jacks. These allow you to send voltage out signals to other equipment.)

I initially tried to connect the Japanino A0 analog pin to the H pin of the output speaker jack, with GND to the G pin of the jack, but the scope sketch wasn’t registering any kind of signal. Instead, I connected A0 to the leftmost pin of the R56 control (VCO1/2 Mixer), and that gave me a large enough signal to work with, without threatening to damage the Japanino. I think having straight numbers out on the LCD shield speeds up the tuning process a lot.


(Bottom of the box. The bottom panel has gaps at the front and back for cooling air flow. The cutout in the upper right corner lets you get at the DIP switch for setting the MIDI port number.)

I think I’ve got the thing tuned for my purposes. There are a few mods on the internet that I may add later, so if I do go back and open the case up again to put in a mod or two, I can always do some retuning if necessary. The next step is to just find out what it can do as-is.)

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